Comets

Draft Dec 11, 2006, updated July 19, 2009

Ancient Ideas about comets

• kometes = `the hairy one” (hairy star)

• 550 BC Pythagoreans thought they were wandering planets.

• Aristotle (350 BC) thought that, like meteors, they were phenomenon of the upper atmosphere. This view dominates for 2000 years.

• 1577 AD, Tycho Brahe shows that a bright comet had no measurable parallax, hence must be at least 4x further away than the moon.

Aristotle

Tycho

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Great Comet of 1577, Prague, Nov 12 3

Comets Orbit the Sun

• 1607 Sir William Lower accurately measures positions of what we now call “Halley’s Comet”. In 1610 suggests orbits are ellipses (?)

• 1618 Cysat: First to observe structure of comet & evolution. Suggests orbit sun in parabola shape

• 1665 Borelli is first to accurately describe comet orbit as an ellipse

Cysat

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Great Comet of 1680

• First to be discovered by telescope (by Kirch)

• 1681 Doefel uses data to show orbit is nearly parabolic

• 1687 Newton shows his theory of gravity predicts ellipses/parabolas and fits the orbit exactly

Kirch

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Edmund Halley (1656-1742)

• 1705 proposes that the comets of 1431, 1531, 1607 and 1682 are the same comet, returning every 76 years

• Predicts return in 1758-9

• Comet indeed appears (16 years after his death). Now called “Halley’s Comet”

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Orbit of Halley’s Comet

It goes outside the orbit of Neptune (probably originally captured by Neptune)

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Orbit of Halley’s Comet

Its retrograde, and inclined by 18, so unlikely to hit the earth as it crosses our orbit

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Short Period Comets

Comets with periods around 200 years or less probably come from Kuiper Belt (e.g. Halley’s Comet)

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Long Period Comets

Comets with periods around million years or less probably come from the Oort Cloud (500 to 50,000 AU)[e.g. Hale-Bopp]

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A long period comet can be changed into a short one if it passes near one of the planets

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Most comets look like this …12

Comet NEAT, showing dust Tail

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Comet Bradfield, showing dust Tail

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Fig 12-18, p.279

Tail can be over 1AU longbut has very little mass

Comet’s dust TAIL always points away from sun15

A tale of two tails

ION TAIL (top, blue): ionized gas pushed away from the Sun by the solar wind.

DUST TAIL (bottom, white): fine dust pushed away from the Sun by radiation pressure.

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A tale of two tails17

A comet is a dusty chunk of ice that partiallyvaporizes as it passes near the Sun

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The parts of a comet:Nucleus, Coma, and Tail

Halley’s Comet in 1986

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As a comet approaches the Sun, its icy nucleus develops a luminous coma, surrounded by a vast hydrogen envelope

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The Nucleus of a Comet 21

What the nucleus is made of

• Ices– H2O, Carbon Monoxide (CO), Methanol (CH3OH), and

other frozen materials• Minerals

– Silicate minerals• Complex organic molecules and dust particles

– C = carbon– H = hydrogen– O = oxygen– N = nitrogen

• This is the “dirty snowball” concept

CHON particles

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The Coma• The coma is the cloud of gas and dust

that evaporates from the nucleus when the comet is close to the Sun and warms up.

A nucleus only 1 mile in diameter can produce a coma

that is 100,000 miles in diameter

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Fig 12-14, p.278

Comet tails can be as much as 100 million miles in length (~ 1 AU)

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Three Adventures with Comets

• Comet Shoemaker-Levy 9 collides with Jupiter (July 16-22, 1994)

• NASA’s Stardust mission collects comet dust from Comet Wild-2 (January 2, 2004)

• NASA’s Deep Impact mission collides with Comet Tempel 1 (July 4, 2005)

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Stardust mission to collect comet dust

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Stardust mission to collect comet dust

• Comet Wild-2 is an ordinary, short-period comet

• The spacecraft approached the comet and collected dust from the coma

• The capsule containing the dust was returned back to Earth

• It landed in the Utah desert in January, 2005, to be picked up for analysis of the dust grains

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The Nucleus of Comet Wild-231

The capsule landed in Utah in January, 2005 32

The dust was captured in Aerogel

in January, 2004

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The dust was captured in Aerogel

in January, 2004

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What are we learning from Stardust?

-The comet’s nucleus has a surprising amount of minerals that formed in a high temperature environment (but comets are COLD in the outer solar system!). Material formed closer to our Sun or around another star altogether.

-The 4.5 billion-year-old comet sulfides (sulfides are key to life).

-The comet also contains iron, sulfides, glassy materials, and olivine.

-Carbon bearing compounds are present as well.

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Comparison of comets we have seen up close

Comet Halley

Comet Borrelly

CometTempel 1

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NASA’s Deep Impact Mission

Impact on Comet Tempel 1July 4, 2005

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• Deep Impact mission was launched in January, 2005

• Impactor (820 pounds) hit the comet at velocity 10 km/sec (~6 miles/sec)

• The energy of the collision vaporized the Impactor

• Impactor took images up to 3 sec before the collision

• The fly-by spacecraft took images before, during, and after the impact

• The fly-by spacecraft was the re-targeted to another comet

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View of Comet Tempel 1 a few moments before impact

Note the craters, ridges, and the entirely unexpected smooth regions (maybe flows of ice)

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A few moments after

impact

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The comet and the spray of debris released by the impact

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Pictures of the comet before and after the impact by the spacecraft.

Pictures from Hubble Space Telescope

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What is being learned?

• The comet is not a rigid solid body• The impact occurred in a “powdery” layer• A crater about 200 yards across was formed • The ejected material made the comet brighter,

but it dissipated after about two days• We’re waiting to hear about the composition of

the comet’s surface and the material ejected by the impact

• This comet has an appearance quite different from the other three comets we have examined up close

• The impact-induced activity subsided after about 4 days

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Comet Shoemaker-Levy 9 collides with Jupiter (July 16-22, 1994)

• In 1993, a cluster of comets was discovered on a collision course with Jupiter

• The cluster had originally been a single comet, but an

earlier close pass by Jupiter broke it up into dozens of pieces

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Here’s what happened …

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The comet swarmheads for impact

on Jupiter

July 1994

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Scars in Jupiter’s Atmosphere

The irregular darksmudges are thesites in Jupiter’satmosphere where two fragments ofthe comet impacted

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The heat from JupiterHot spots from the impact of comet fragments

July 1994

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The impact events werepredicted a year in advance.

They were observed byastronomers world-wide.

For the first time ever,we had the opportunityto observe collisionsbetween Solar Systembodies. 20 of them !!!

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